Integrated dilution refrigerators

ABSTRACT

A dilution refrigerator is provided. The dilution refrigerator includes an outer vacuum chamber comprising at least one substantially planar surface and an opening in the at least one substantially planar surface configured to provide access to an interior of the outer vacuum chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 63/319,248 filed on Mar. 11, 2022,and titled “INTEGRATED DILUTION REFRIGERATORS,” and of U.S. ProvisionalPatent Application No. 63/219,795 filed on Jul. 8, 2021, and titled“INTEGRATED DILUTION REFRIGERATORS,” the contents of each of which areincorporated by reference herein in their entirety.

BACKGROUND

Dilution refrigerators are cryogenic devices that rely on the heat ofmixing of the ³He and ⁴He isotopes to provide cooling down totemperatures between approximately 2 mK and 1 K. Classic dilutionrefrigerators, or “wet” dilution refrigerators, precool the ³He/⁴Hemixture using liquid nitrogen and ⁴He baths before further cooling ofthe ³He/⁴He mixture below 4 K. Modern dilution refrigerators, or “dry”dilution refrigerators, precool the ³He/⁴He mixture using devices suchas a cryocooler rather than cryogenic liquid baths.

SUMMARY

Some embodiments are directed to a dilution refrigerator comprising: anouter vacuum chamber comprising at least one substantially planarsurface; and an opening in the at least one substantially planar surfaceconfigured to provide access to an interior of the outer vacuum chamber.

In some embodiments, the dilution refrigerator further comprises asample stage housed within the outer vacuum chamber, wherein the openingis configured to provide access to the sample stage through the outervacuum chamber.

In some embodiments, the dilution refrigerator further comprises one ormore radiation shields housed within the outer vacuum chamber, andwherein portions of the one or more radiation shields proximate thesample stage are slidable and/or removable to provide access to thesample stage.

In some embodiments, the at least one substantially planar surfacecomprises a first surface disposed within a plane perpendicular to aplane of a floor supporting the dilution refrigerator.

In some embodiments, the at least one substantially planar surfacecomprises: first surfaces disposed within a plane perpendicular to aplane of a floor supporting the dilution refrigerator; and secondsurfaces disposed within a plane parallel to the plane of the floor,wherein the first surfaces and the second surfaces are arranged as arectangular prism.

In some embodiments, the opening comprises a hermetic opening.

In some embodiments, the opening comprises a hinged door.

In some embodiments, the dilution refrigerator further comprises one ormore radiation shields housed within the outer vacuum chamber, andwherein portions of the one or more radiation shields proximate thehinged door are slidable and/or removable to provide access to theinterior of the outer vacuum chamber.

In some embodiments, the opening further comprises a load lock.

In some embodiments, the outer vacuum chamber comprises: a firstsection; and a second section suspended from the first section.

In some embodiments, the first section is coupled to the second sectionby integrated clamps and/or cams.

In some embodiments, the first and/or second section of the outer vacuumchamber are configured to be at least partially removed from thedilution refrigerator to provide access to an interior of the outervacuum chamber.

In some embodiments, the dilution refrigerator further comprises anexternal system configured to lift and/or lower the first and/or secondsection of the outer vacuum chamber.

In some embodiments, the external system comprises pneumatic and/orhydraulic devices configured to lift and/or lower the first and/orsecond sections.

In some embodiments, the external system comprises a screw mechanismconfigured to lift and/or lower the first and/or second sections.

In some embodiments, the outer vacuum chamber is configured to fitwithin a server rack-type container.

In some embodiments, the server rack-type container is configured tointegrate with commercial server rack infrastructure.

In some embodiments, the server rack-type container is a 19-inch serverrack.

In some embodiments, the server rack-type container comprises anexternal housing comprising an integrated horizontal surface.

In some embodiments, the integrated horizontal surface is configured tobe stowed when not in use.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic diagram of a closed-cycle dilution refrigerator,in accordance with some embodiments described herein.

FIG. 2 is a schematic diagram of helium cleaning devices in the dilutionrefrigerator of FIG. 1 , in accordance with some embodiments describedherein.

FIG. 3 is a schematic diagram of a cooldown turbo charger device in thedilution refrigerator of FIG. 1 , in accordance with some embodimentsdescribed herein.

FIG. 4 is a schematic diagram of a still including a device to separate³He and ⁴He using second sound effects, in accordance with someembodiments described herein.

FIG. 5A shows illustrative components of a removable dilution insert forthe dilution refrigerator of FIG. 1 , in accordance with someembodiments described herein.

FIG. 5B is a view of an illustrative thermalization plate insert of theremovable dilution insert of FIG. 5A, in accordance with someembodiments described herein.

FIG. 5C is a cross-sectional view of an illustrative integrated heatexchanger showing channels for helium flow, in accordance with someembodiments described herein.

FIG. 5D is an image of a high surface area material for use in anintegrated heat exchanger, in accordance with some embodiments describedherein.

FIG. 5E is an illustrative implementation of a continuous heat exchangerand discrete heat exchanger of the dilution refrigerator of FIG. 1 , inaccordance with some embodiments described herein.

FIGS. 6A-6F are schematic diagrams of illustrative ⁴He film separationdevices, in accordance with some embodiments described herein.

FIG. 7A is an image of sintered metal particles for use in a heatexchanger.

FIG. 7B is an image of nanowires for use in a heat exchanger, inaccordance with some embodiments described herein.

FIG. 7C is an image of a nanocluster for use in a heat exchanger, inaccordance with some embodiments described herein.

FIG. 7D includes images of different nanopellets for use in a heatexchanger in accordance with some embodiments described herein.

FIG. 8A is a schematic diagram of an illustrative vibration isolationsystem for use in the dilution refrigerator of FIG. 1 , in accordancewith some embodiments described herein.

FIG. 8B shows an illustrative spring for use in the vibration isolationsystem of FIG. 8A, in accordance with some embodiments described herein.

FIG. 9 is a side view of an illustrative external support rack andintegrated lift configured to raise and lower portions of the vacuumchamber, in accordance with some embodiments described herein.

FIG. 10A is a side view of an illustrative implementation of thedilution refrigerator of FIG. 1 including elements configured to providetool-free assembly of the vacuum chamber and access to the experimentalvolume, in accordance with some embodiments described herein.

FIGS. 10B and 10C are views of integrated latches in open and closedpositions, respectively, configured to provide tool-free assembly of thevacuum chamber, in accordance with some embodiments described herein.

FIG. 11 is an exterior view of a housing configured to support adilution refrigerator, in accordance with some embodiments describedherein.

FIG. 12A is a schematic diagram of an inverted dilution refrigerator, inaccordance with some embodiments described herein.

FIGS. 12B and 12C are schematic diagrams of illustrative components ofan inverted dilution refrigerator, in accordance with some embodimentsdescribed herein.

FIG. 13 is a schematic diagram of a distributed cooling system, inaccordance with some embodiments described herein.

FIG. 14 depicts, schematically, an illustrative computing device onwhich aspects of the technology described herein may be implemented.

DETAILED DESCRIPTION

Dilution refrigerators are cryogenic devices that can provide coolingdown to temperatures between approximately 2 mK and 1 K and are used ina variety of applications requiring these extremely low temperatures.For example, dilution refrigerators can be used to support quantumcomputing (e.g., superconducting quantum computing technologies andqubits) and low-temperature condensed matter physics research, amongother applications.

As described above, dilution refrigerators rely on the heat of mixing of³He and ⁴He isotopes to provide cooling. When cooled below approximately870 mK, a ³He/⁴He mixture undergoes spontaneous phase separation to forma ³He-rich phase (the “concentrated” phase) and a ³He-poor phase (the“dilute” phase). These two phases are maintained in equilibrium in amixing chamber, the coldest part of the dilution refrigerator, and areseparated by a phase boundary. In the mixing chamber, the ³He is dilutedas it moves from the concentrated phase through the phase boundary intothe dilute phase, and the heat necessary for this endothermic dilutionprocess provides the cooling power of the dilution refrigerator.

However, conventional dilution refrigerators can suffer from a multitudeof drawbacks and failure points. For example, wet dilution refrigeratorsrequire significant amounts of liquid cryogens, which are costly tomaintain and supply. As another example, dry dilution refrigerators canbe subject to unwanted mechanical vibrations introduced by thecryocooler system and/or may draw large amounts of energy to power thecryocooler.

Conventional dilution refrigerators also typically occupy a largefootprint, which may be prohibitive for applications requiring multipledilution refrigerators. For example, a single conventional dry dilutionrefrigerator typically requires approximately 300 square feet andceiling heights between 12 and 14 feet. This space is occupied not onlyby the dilution refrigerator itself but is also required to support anyauxiliary systems such as pumps, compressors, water cooling systemsand/or cryocooler systems.

The inventors have recognized and appreciated that, for quantumcomputing and other quantum technologies to be easily scalable, thequantum technology industry needs reliable, easy-to-use,easy-to-maintain, and compact dilution refrigerators. Accordingly, theinventors have developed dilution refrigerators and distributed coolingsystems that can be integrated with commercially available server rackinfrastructure (e.g., 19-inch server racks). Additionally, the inventorshave developed a number of features, described herein, to easemaintenance of the dilution refrigerators, speed cooling of the dilutionrefrigerators without the use of mechanical pumps, and to reduce thetransmission of mechanical vibrations to the experimental volume of thedilution refrigerator.

I. Improved Closed-Cycle Dilution Refrigerator

FIG. 1 is a schematic diagram of a dry, closed-cycle dilutionrefrigerator 100, in accordance with some embodiments described herein.In some embodiments, the dilution refrigerator 100 includes an outervacuum chamber 106 at room temperature (e.g., approximately 300K) and anumber of thermal stages 108 a-108 f (e.g., thermalization plates) heldat decreasing temperature intervals (e.g., approximately 50 K, 9-10 K, 3K, etc.). For example, the first thermal stage 108 a may be atapproximately 50 K, the second thermal stage 108 b may be atapproximately 9-10 K, the third thermal stage 108 c may be atapproximately 3-4 K, the fourth thermal stage 108 d may be atapproximately 800 mK, the fifth thermal stage 108 e may be atapproximately 100 mK, and the sixth thermal stage 108 f may be atapproximately 10 mK.

In some embodiments, the dilution refrigerator 100 may include a pumpsystem 102 that pressurizes a ³He/⁴He gas mixture (e.g., to a pressureat or near 1 bar). The ³He/⁴He gas mixture may enter the outer vacuumchamber 106 through one or more inlets and thereafter may travel throughthe inner thermal stages 108 a-108 f through the condensing line 102 a.After performing its cooling function, the ³He/⁴He mixture may return tothe pump system through the return 102 b.

In some embodiments, the ³He/⁴He mixture may be purified prior totraveling along the condensing line 102 a through the thermal stages 108a-108 f. Contaminants in the helium flowing through the dilutionrefrigerator can clog certain components (e.g., the Joule-Thomsonexpander or capillaries in the heat exchangers) and lead to performancedegradation or system failure. Conventionally, to reduce the risk ofcontaminants making their way into the system, helium is first passedthrough an external ‘cleaning trap’ filled with activated charcoalbefore entering the dilution unit of the dilution refrigerator. Theseexternal traps must be surrounded by liquid nitrogen and refilled atfrequent intervals, which requires user maintenance and interaction.

The inventors have recognized and appreciated that passive heliumfilters, without the need for refilling of liquid nitrogen, can improvethe user experience and reduce maintenance frequency of a dilutionrefrigeration system. Accordingly, in some embodiments, the dilutionrefrigerator 100 includes one or more helium cleaning devices 110. Insome embodiments, where the dilution refrigerator 100 includes two ormore helium cleaning devices 110, the dilution refrigerator 100 mayfurther include a switching system 109 configured to direct the flow ofthe helium to a single helium cleaning device 110.

FIG. 2 shows a schematic diagram of helium cleaning devices 110, inaccordance with some embodiments described herein. The helium cleaningdevices 110 may be coupled to the pump system 102 by a switching system109 that is disposed outside of the outer vacuum chamber 106. Theswitching system 109 may include one or more helium-compatible valves.The switching system 109 may be configured to switch helium flow betweeneach of the helium cleaning devices 110. In this manner, one heliumcleaning device 110 may be used for actively filtering helium while thedilution refrigerator is in operation while the other helium cleaningdevice may be cleaned (e.g., by heating and pumping out of impurities),enabling indefinite periods of operation of the dilution refrigerator100.

In some embodiments, the helium cleaning devices 110 include a counterflow heat exchanger 110 a, a trap 110 b, and a weak thermal contact 110c (e.g., a gas gap heat exchanger, a low thermal conductivityattachment, etc.). The counter flow heat exchanger 110 a and the weakthermal contact 110 c may reduce the thermal load of the helium cleaningdevices 110 on the dilution refrigerator 100 and may eliminate the useof cryogenic valves in the helium cleaning devices 110. The trap 110 bmay include, for example, a high surface area material (e.g., charcoal,activated charcoal, and/or a metal powder) configured to capturenon-helium impurities in the dilution refrigerator 100.

Returning to FIG. 1 , in some embodiments, the dilution refrigerator 100may include a cooldown turbo charger device 111. Dry dilutionrefrigerators conventionally use an auxiliary compressor to enable theflow of warm helium, which initially has a high impedance and resistssuch movement. The extra pressure from the auxiliary compressor alsopressurizes the helium, causing the helium to reach a pressure thatstarts isenthalpic expansion and cooling at a higher temperature. Theseauxiliary mechanical compressor pumps are costly, prone to reliabilityissues, frequently leak, and can cause performance degradation overtime. The inventors have recognized and appreciated that, alternatively,the helium may be pulsed through the dilution refrigerator duringcooldown without the use of an auxiliary mechanical pump, enabling afaster and more efficient cooldown process.

FIG. 3 shows a schematic diagram of a cooldown turbo charger device 111,in accordance with some embodiments described herein. The cooldown turbocharger device 111 may include a volume of a high surface area material111 a, a heater 111 b, a first valve 111 c, and a second valve 111 d.The first and second valves 111 c, 111 d may be, for example, coldvalves located inside the vacuum chamber 106. As another example, thefirst and second valves 111 c, 111 d may be room temperature valveslocated outside of the vacuum chamber 106. In some embodiments, theheater 111 b and the first and second valves 111 c, 111 d may becommunicatively coupled to a controller 330. The controller 330 may be,for example, a computer as described in connection with FIG. 14 herein.

In some embodiments, the cooldown turbo charger device 111 may bethermally coupled to a thermal stage (e.g., to a thermalization plate).In the example of FIG. 3 , the high surface area material 111 a isthermally coupled to the second thermal stage 108 b, though it should beappreciated that the high surface area material 111 a could be thermallycoupled to another thermal stage in some embodiments (e.g., firstthermal stage 108 a).

Alternatively, in some embodiments, the cooldown turbo charger device111 may be thermally coupled to multiple thermal stages (e.g., acrosstwo or more thermal stages 108 a-108 f). In such embodiments, thesequential heating and cooling of the high surface area material 111 amay be mediated by heat switches. For example, the cooldown turbocharger device 111 may be switchably thermally coupled between a warmerthermal stage and a colder thermal stage such that the cooldown turbocharger device 111 may be thermally coupled to either the warmer thermalstage or the colder thermal stage. When the cooldown turbo chargerdevice 111 is thermally coupled to the warmer thermal stage, the highsurface area material 111 a may release any adsorbed helium. When thecooldown turbo charger device 111 is thermally coupled to the colderthermal stage, helium may begin adsorbing to the high surface areamaterial 111 a. In this manner, a sequential flushing of helium throughthe condensing line 102 a may be implemented.

In some embodiments, the high surface area material 111 a may comprise amaterial with a porous and/or textured surface such that helium adsorbsto the high surface area material 111 a during the cooldown process. Forexample, the high surface area material 111 a may comprise activatedcharcoal, a metal powder (e.g., a copper or silver powder), and/or amaterial composite formed of nanostructures (e.g., nanowires,nanoparticles, etc.).

In some embodiments, the cooldown turbo charger device 111 may beoperated using a sequential opening and closing of the valves 111 c, 111d in concert with operation of the heater 111 b. For example, to causehelium to adsorb the high surface area material 111 a, the first valve111 c may be closed to prevent helium from flowing to the lower stagesof the dilution refrigerator 100 and the second valve 111 d may beopened to allow helium to reach the high surface area material 111 a.The first valve 111 c may be closed and the second valve 111 d may beopened by the controller 330 in response to a measured pressure ortemperature or in response to a timing signal generated by controller330.

In some embodiments, after sufficient helium has adsorbed onto the highsurface area material 111 a, the second valve 111 d may be closed andthe first valve 111 c may be opened. The first and second valves 111 c,111 d may be opened and/or closed in response to measured temperaturesor pressures and/or in response to a timing signal generated bycontroller 330.

In some embodiments, when the second valve 111 d is closed and the firstvalve 111 c is opened, the heater 111 b may also be caused, at a same orsimilar time, to heat the high surface area material 111 a in responseto a signal generated by controller 330. For example, the heater 111 bmay be a resistive heater that is caused to heat the high surface areamaterial 111 a by a flow of current through the heater 111 b. Inresponse to the heat from the heater 111 b, the helium adsorbed to thehigh surface area material 111 a may act as a reserve that is thenreleased from the high surface area material 111 a. This release of theadsorbed helium may increase the pressure in the remainder of thecondensing line 102 a, and the increased pressure may enable the startof isenthalpic expansion to speed cooling of the dilution refrigerator100.

In some embodiments, once the helium has been released from the highsurface area material 111 a, the first valve 111 c may be closed, thesecond valve 111 d may be opened, and the heater 111 b may be turned offby the controller 330, allowing new helium to adsorb to the high surfacearea material 111 a. The controller 330 may be configured toperiodically (e.g., at regular time intervals, at irregular timeintervals, at time intervals determined by the temperature of theexperimental volume, at time intervals determined by the pressure of theexperimental volume) open and/or close the valves 111 c, 111 d and tooperate the heater 111 b to flush the helium intake path. In someembodiments, the controller 330 may be configured to “pulse” the heliumfrom the high surface area material 111 a through the condensing line102 a, causing the dilution refrigerator to be cooled.

Returning to FIG. 1 , during operation of the dilution refrigerator, the³He/⁴He mixture may be progressively cooled as it travels along thecondensing line 102 a from the first thermal stage 108 a to the mixingchamber 122. At the first thermal stage, the helium may be initiallycooled to approximately 50 K. After exiting the cooldown turbo chargerdevice, the ³He/⁴He mixture may next be cooled by a cryocooler 104. Aportion of the cryocooler 104 may be disposed partially outside of theouter vacuum chamber 106, in some embodiments. The cryocooler 104 may bevibrationally isolated from outer vacuum chamber 106 by a vibrationisolation stage 105, which may comprise padding and/or any othersuitable vibration isolation techniques.

In some embodiments, the cryocooler 104 may be coupled to a cryocoolersupport 103. The cryocooler support 103 may be, for example, acompressor and/or compression system, in some embodiments. Thecryocooler support 103 may include cooling members 103 a, in someembodiments, configured to provide air-cooling to the dilutionrefrigerator 100. The cooling members 103 a may be, as a non-limitingexample, cooling fins, fans, and/or heat pipes configured to removewaste heat generated by the cryocooler support 103 and/or the cryocooler104.

The cooling members 103 a are in contrast to conventional closed-cycledilution refrigerators, which typically rely on water-cooling to removewaste heat generated by the integrated cryocooler. Water-cooling of thecryocooler, however, requires installing a large and/or expensivewater-cooling system in conjunction with the dilution refrigerator.Additionally, such water-cooling systems are not typically integratedwith commercial computing facilities, which typically rely onair-cooling as it is less expensive and does not present hazards (e.g.,leaking coolant, flooding, etc.) to the electronic equipment. Theinventors have accordingly recognized that using air-cooling to removeheat from the cryocooler of the dilution refrigerator may reduce thecosts of manufacturing dilution refrigerators and enable their use incommercial computing facilities.

In some embodiments, the dilution refrigerator 100 may be disposed abovea plenum (not pictured) disposed under a floor supporting the dilutionrefrigerator. The plenum may supply the cooling members 103 a with airflow to provide air-cooling. In some embodiments, the cooling members103 a may include inlets and/or louvers configured to draw in air fromthe plenum. Alternatively or additionally, in some embodiments, thedilution refrigerator 100 may be disposed in a facility includingductwork and/or heat pipes (not pictured) arranged to remove heat fromthe cooling members 103 a, the cryocooler support 103, and/or thecryocooler 104 and to minimize vibrations experienced by the dilutionrefrigerator 100.

In some embodiments, the ³He/⁴He mixture may be cooled by the cryocooler104 in two steps. The condensing line 102 a may be wound around twoportions of the cryocooler 104 to effect heat exchange between the³He/⁴He mixture in the condensing line 102 a and the cryocooler 104. Ina first step, the ³He/⁴He mixture may be cooled to approximately 10 K bythe cryocooler 104. In a second step, the ³He/⁴He mixture may be cooledto approximately 3-4 K by the cryocooler 104.

In some embodiments, after being cooled by the cryocooler 104, the³He/⁴He mixture may pass through the third thermal stage 108 c. Thethird thermal stage 108 c may be thermally coupled but mechanicallydecoupled from cryocooler 104, in some embodiments, in order to providevibration isolation to the later thermal stages 108 d-108 f. As anon-limiting example, in some embodiments, third thermal stage 108 c maybe mechanically decoupled from the cryocooler by a copper braid, heatstrap, or other hanging component configured to maintain thermalcoupling between the third thermal stage 108 c and the cryocooler 104.

In some embodiments, after passing through the third thermal stage 108c, the ³He/⁴He mixture may enter a primary impedance stage 112. Theprimary impedance stage 112 may be a Joule-Thomson expander configuredto reduce the temperature and/or pressure of the ³He/⁴He mixture. Forexample, in some embodiments, the ³He/⁴He mixture may be atapproximately 3-5 K before entering the primary impedance stage 112 andmay be at approximately 1 K after exiting the primary impedance stage112.

In some embodiments, the primary impedance stage 112 may be aJoule-Thomson expander formed from a fiber optic cable. Conventionally,Joule-Thomson expanders may be formed as metal tubes that aremanufactured by pulling. However, such metal Joule-Thomson expanders maysuffer from irregularities and/or may have a larger diameter thatreduces the cooling power of the device. A hollow-core fiber optic cablemay reliably and reproducibly provide the narrow opening needed for aJoule-Thomson expander.

In some embodiments, the dilution refrigerator 100 may include a bypassdevice 113 a configured to increase the speed of cooldown of thedilution refrigerator 100. For example, during the initial cooldown of adilution refrigerator, the helium flow rate may be low due to the largeimpedance of Joule-Thomson expanders in the dilution refrigerator andthe warm, low density, and viscous circulating helium. This reducedhelium flow rate reduces the rate of cooling of the lower portions ofthe refrigerator. To combat this effect, conventionally, a needle valvemay be incorporated at a location on the condensing line above theJoule-Thomson expander to reduce the impedance of the initially-warmhelium gas. However, the needle valve includes mechanical componentsthat may fail over time. The inventors have recognized and appreciatedthat the helium flow rate may be improved without reliance on amechanical component such as a needle valve.

In some embodiments, the bypass device 113 a may be disposed along thecondensing line 102 a in parallel with the primary impedance stage 112and on a bypass line 113 b that bypasses the primary impedance stage 112(e.g., allowing the ³He/⁴He mixture to flow around the primary impedancestage 112). The bypass device 113 a may include a sheet ofvacuum-compatible material configured to allow helium to diffuse throughthe material at temperatures above a threshold temperature value. Forexample, the bypass device 113 a may allow the ³He/⁴He mixture todiffuse through the bypass device 113 a at temperatures in a range fromapproximately 40 K to 300 K, in a range from 50 K to 300 K, in a rangefrom 80 K to 300 K, in a range from 100 K to 300 K, in a range from 150K, or in any range within those ranges. In some embodiments, the bypassdevice 113 a may include a sheet of a vacuum-compatible polymermaterial. For example, the bypass device 113 a may be formed of a sheetof Kapton, PEEK, and/or mylar, as some non-limiting examples. The bypassdevice 113 a therefore allows for the high impedance of the primaryimpedance stage 112 to be circumvented when the ³He/⁴He mixture is warm,thereby increasing the helium flow rate and rate of cooling of thedilution refrigerator 100. When the dilution refrigerator 100 has cooledsufficiently (e.g., to below the threshold temperature value), the³He/⁴He mixture will no longer diffuse through the bypass device 113 aand instead will flow through the primary impedance stage 112.

In some embodiments, after exiting the primary impedance stage 112 orthe bypass device 113 a, the ³He/⁴He mixture then travels past thefourth thermal stage 108 d and into the still 114. The still 114 maycontain a different mixture of liquid ³He/⁴He that cools the incoming³He/⁴He mixture as it passes through the condensing line 102 a runningthrough the still 114. In some embodiments, the ³He/⁴He mixture in thecondensing line may be cooled to approximately 400-900 mK by the still114.

In some embodiments, the still 114 may include a membrane configured touse second sound to improve ³He evaporation within the still. Secondsound is a superfluid phenomenon present in superfluid helium and may beproduced, for example, when a porous membrane is oscillated or a heatedwire is cycled within a bath of superfluid helium. The two-fluid modelspecifies that the superfluid helium in the mixture moves through themembrane while non-superfluid components of the helium bath cannot passthrough the porous membrane as easily. In superfluid helium, thiscreates an enthalpy or temperature wave. Analogously in helium mixtures,the non-superfluid ³He may be preferentially pushed by the oscillatingmembrane while the superfluid ⁴He remains relatively stationary. Theinventors have recognized and appreciated that this second soundphenomenon can be implemented within the still 114 to increase the ³Heevaporation rate at a lower temperature and to reduce a concentration of⁴He in the vapor above the liquid helium mixture in the still.

FIG. 4 is a schematic diagram of a still 400 including a device toseparate ³He and ⁴He using second sound effects, in accordance with someembodiments described herein. The still 400 may be implemented as still114 of dilution refrigerator 100, in some embodiments. The still 400 mayinclude a stationary surface 402 and a porous, movable membrane 404. Theporous membrane 404 may be oscillated along the Z direction to createstanding concentration waves of ³He within the still so that there areregions 406 of lower ³He concentration and regions 408 of high ³Heconcentration. The standing wave may be tuned such that regions 408 ofhigh ³He concentration are disposed adjacent an outlet of the still,thereby allowing for improved ³He purification.

Returning to FIG. 1 , in some embodiments, after exiting the still 114,the ³He/⁴He mixture may flow through the condensing line 102 a to asecondary impedance stage 116. The secondary impedance stage 116 may beconfigured to ensure that only liquid ³He/⁴He proceeds furtherdownstream in the dilution refrigerator 100 and that gas cavitation inthe still 114 does not occur (e.g., by maintaining a threshold pressurein the still 114). The secondary impedance stage 116 may thereforereduce downstream cooling loads due to a latent heat of gaseous ³He/⁴He.

In some embodiments, after exiting the secondary impedance stage 116,the ³He/⁴He mixture may then flow into a first heat exchanger 118. Thefirst heat exchanger 118 may be a continuous heat exchanger. Forexample, the first heat exchanger 118 may be a counterflow (e.g., atube-tube heat exchanger), a cross-counterflow, and/or coflow heatexchanger. At the exit of the first heat exchanger 118, the ³He/⁴Hemixture in the condensing line 102 a may be cooled to a temperature ofapproximately 20 mK.

In conventional closed-cycle dilution refrigerators, prior to enteringthe still, the ³He/⁴He mixture in the condensing line typically mustpass through a first impedance stage. This first impedance stagetypically acts as an independent refrigeration stage, known as aJoule-Thomson refrigerator, where the ³He/⁴He mixture is cooled byisenthalpic expansion. In order to control the cooling power of the³He/⁴He mixture expansion, the pressure of the ³He/⁴He mixture in thecondensing line is typically raised by an external compressor.

The inventors have recognized and appreciated that reducing thetemperature of the ³He/⁴He mixture prior to entering the first impedancestage can achieve the same cooling effect (e.g., the ³He/⁴He mixture canreach the same base temperature after passing through the firstimpedance stage) while using a lower pressure differential. Such aconfiguration can improve the efficiency of the dilution refrigeratorand reduce or eliminate the need to pressurize the ³He/⁴He mixturebefore the first impedance stage. Further, the inventors have recognizedthat the heat removed from the ³He/⁴He mixture prior to the firstimpedance stage can be returned to the still, thereby eliminating orreducing the need for a supplemental heater within the still to raisethe vapor pressure, and enabling evaporation, of the different ³He/⁴Hemixture present in the still.

Accordingly, in some embodiments, the dilution refrigerator 100 mayinclude a heat exchange line 117 configured to transfer heat from theincoming condensing line 102 a to a return helium mixture beingtransported from the first heat exchanger 118 to the still 114. The heatexchange line 117 may cool the condensing line 102 a at a location abovethe primary impedance stage 112. The heat exchange line 117 then coolsthe ³He/⁴He mixture in the condensing line 102 a prior to entering theprimary impedance stage 112. Thereafter, the warmed mixture in the heatexchange line 117 is transported to the still 114.

Cooling the incoming ³He/⁴He mixture before it enters the primaryimpedance stage 112 causes the primary impedance stage 112 to output a³He/⁴He mixture with a higher proportion of ³He in the liquid staterather than in the vapor state. Thus, the primary impedance stage 112can be made more efficient by including the additional heat exchangeline 117. Additionally, this improved efficiency eliminates or mitigatesthe need for supplemental pressure (e.g., an external compressor) andmay lower the flow impedance of the circulating helium mixture. This isparticularly useful in reducing the complexity and size of smallerdilution refrigerators that include smaller (i.e., less powerful) pulsetubes or other cryocoolers.

In some embodiments, after exiting the first heat exchanger 118, the³He/⁴He mixture passes through the fifth thermal stage 108 e and enterscontinuous heat exchanger 119. The continuous heat exchanger 119 may bea counterflow (e.g., a tube-tube heat exchanger), a cross-counterflow,and/or coflow heat exchanger. The continuous heat exchanger 119 isdisposed below the fifth thermal stage 108 e. The fifth thermal stage108 e may be an intermediate cold plate (ICP) configured to be cooled toa temperature of approximately 100-200 mK. While continuous heatexchangers are typically more efficient than discrete heat exchangers,they become less efficient below a temperature of approximately 80 mK.However, adding continuous heat exchanger 119 below the fifth thermalstage 108 e may enable the fifth thermal stage 108 e to operate withmore cooling power during the process of cooling down the dilutionrefrigerator.

In some embodiments, after exiting the continuous heat exchanger 119,the ³He/⁴He mixture enters discrete heat exchangers 120. The discreteheat exchangers 120 may be formed of sintered nanoparticles, in someembodiments. Alternatively or additionally, in some embodiments thediscrete heat exchangers 120 may be formed of sintered nanowires, asdescribed herein in connection with FIGS. 7A-7D herein. The discreteheat exchangers 120 may be configured to further cool the ³He/⁴Hemixture to a temperature below approximately 10-20 mK.

The inventors have additionally recognized and appreciated that the userexperience may be improved by allowing users to easily swap parts in andout of the dilution refrigerator 100 (e.g., for maintenance, to changethe characteristics of the dilution refrigerator 100, and/or to upgradethe dilution refrigerator 100 as technological innovations aredeveloped). The inventors have accordingly developed a swappabledilution insert that is easily removed and replaced. FIG. 5A showsillustrative components of a removable dilution insert 540 for thedilution refrigerator 100 of FIG. 1 , in accordance with someembodiments described herein.

In some embodiments, the removable dilution insert 540 includesdetachable plates 540 a, 540 b, 540 c removably coupled to thermalstages 108 d, 108 e, and 108 f, respectively. As shown in the example ofFIG. 5A, the detachable plates 540 a, 540 b, and 540 c may be removablycoupled using mechanical fasteners (e.g., bolts and/or screws). In someembodiments, the removable dilution insert 540 further includesdetachable connections 540 d above the still (e.g., flanges) and to thecondensing line 102 a to further simplify replacement of the removabledilution insert 540.

FIG. 5B shows a close-up view of a detachable plate 540 b of theremovable dilution insert of FIG. 5A, in accordance with someembodiments described herein. The detachable plate 540 b includes aninput A and an outlet B to allow helium to flow through the detachableplate 540 b, in some embodiments.

In some embodiments, the detachable plates 540 a, 540 b, and/or 540 cmay include integrated heat exchangers. In some embodiments, theintegrated heat exchangers may be channels 542 formed in the detachableplates 540 a and/or 540 b, as illustrated by the example of FIG. 5C,which shows a cross section through the detachable plate 540 b of FIG.5B. The channels 542 may be configured to have a high surface area. Byallowing helium to flow through the channels 542 in the detachableplates 540 a and/or 540 b, the helium's rate of cooling may beincreased. In some embodiments, the channels 542 may be formed bymachining, welding, and/or by additive fabrication techniques (e.g.,three-dimensional printing techniques).

In some embodiments, the integrated heat exchangers may be a highsurface area material structure formed in the detachable plate 540 c.For example, the integrated heat exchangers may be a lattice structure,as shown in the example of FIG. 5D. The lattice structure may be, as anon-limiting example, a square lattice structure having a periodicity ina range from approximately 400 μm to approximately 1000 μm. For example,the lattice structure may have a periodicity of approximately 600 μm.

In some embodiments, the lattice structure may be fabricated usingadditive fabrication techniques (e.g., three-dimensional printingtechniques). The lattice structure may be fabricated to have a roughsurface texture to increase the surface area of the material in contactwith the helium mixture passing through the integrated heat exchanger,thereby improving heat exchange. In some embodiments, the latticestructure may be formed of a metal. As non-limiting examples, thelattice structure may be formed of copper, silver, and/or aluminum.

In some embodiments, the dilution insert 540 may include one or moreheat exchangers, as described in connection with dilution refrigerator100 of FIG. 1 . FIG. 5E shows an illustrative implementation of acontinuous heat exchanger 119 and discrete heat exchanger 120, inaccordance with some embodiments described herein. As shown in FIG. 55E,the helium flows from the detachable plate 540 b coupled to fifththermal stage 108 e into the continuous heat exchanger 119 followed bythe discrete heat exchanger 120 and the detachable plate 540 c.

Refrigeration cycles in cryogenic coolers typically use a method tocontrol the flow of heat throughout the system. This control may beachieved with a superconductor, a gas gap, or other mechanisms to makeor break a thermal connection between components within the system(i.e., heat switches). One common type of heat switch, a gas gap,typically comprises two high surface area objects with a small gapbetween them that is filled with a gas. As the system falls below acertain temperature, the conductive gas is adsorbed onto surface area inthe heat switch, creating a vacuum and reducing heat transfer betweenthe surfaces. Another common type of heat switch is a superconductingswitch where a material passes through a superconducting transition andreduces thermal conductivity.

In some embodiments, the dilution refrigerator 100 may further includecombined gas gap and/or superconducting heat switches between thermalstages of the dilution refrigerator 100. The example of FIG. 5E showssuch a combined gas gap and superconducting heat switch 550 thermallycoupled between thermal stages 108 e and 108 f. The combined gas gap andsuperconducting heat switches 550 include both a superconductingmaterial (e.g., aluminum, titanium) that becomes superconducting at atemperature higher than a target temperature of the thermal stage towhich it is thermally coupled and a gas gap heat switch to improvethermal isolation between the thermal stages.

Returning to FIG. 1 , after the ³He/⁴He mixture exits the discrete heatexchanger 120, it passes through the sixth thermal stage 108 f andenters the mixing chamber 122. In the mixing chamber 122, ³He atoms maybe pumped from a concentrated phase into a dilute phase (i.e., mixedwith ⁴He). This mixing causes the ³He to be cooled as it passes throughthe phase transition between the concentrated phase to the dilute phase,and this endothermic phase transition provides the final cooling powerof the dilution refrigerator 100.

In some embodiments, an experimental volume 124 (e.g., a sample stage orplate) may be thermally coupled to the mixing chamber 122 and configuredto support a sample and/or quantum device. Because the experimentalvolume 124 is thermally coupled to the mixing chamber 122, the sampleand/or quantum device may be held at or near the mixing chambertemperature.

In some embodiments, the experimental volume 124 may be accessed by theuser when the dilution refrigerator 100 is not in operation through anopening in the vacuum chamber 106 and door 125. The door 125 may be, insome embodiments, a removable panel (e.g., secured with mechanicalfasteners) or may be a hinged door that a user may open using a clampedhandle (e.g., as shown in the example of FIG. 10A herein).

As illustrated in the example of FIG. 1 , certain components of thedilution refrigerator 100 may be thermally coupled to a thermal stageand disposed on one side (e.g., above or below) the thermal stage. Forexample, the still 114 is shown as being disposed on a lower surface ofthe fourth thermal stage 108 d. It should be appreciated that in someembodiments, such components may be disposed on either side of theassociated thermal stage, as aspects of the technology described hereinare not limited in this respect. For example, in some embodiments thestill 114 may be disposed on an upper surface of the fourth thermalstage 108 d. As another example, the helium cleaning devices 110 may bedisposed on a lower surface of the first thermal stage 108 a, in someembodiments.

In some embodiments, after entering the dilute phase, the ³He/⁴Hemixture may be pumped out of the mixing chamber 122 and back through thedilution refrigerator 100, exiting the outer vacuum chamber 106 throughreturn 102 b. At low temperatures and pressures, ⁴He forms a thick andmobile film that can move long distances across surfaces, includingmoving in a direction counter to the force of gravity. This helium creepcan result in ⁴He entering portions of a dilution refrigeration systemwhere it is unwanted (e.g., spanning the gap between thermally isolatedareas). FIGS. 6A-6F are schematic diagrams of exemplary ⁴He separationdevices that may be implemented at outlets in, for example, the mixingchamber 122 and/or the still 114, in accordance with some embodimentsdescribed herein.

FIG. 6A illustrates a cooling stage 600 (e.g., a still, the mixingchamber, etc.) including a bath 602 of a dilute phase helium mixture(e.g., a ³He/⁴He mixture), in some embodiments. ⁴He can creep up anoutlet pipe (e.g., a still pumping outlet pipe, a mixing chamber outletpipe) and exit the cooling stage 600 unless a barrier 606 prevents thehelium creep. In the example of FIG. 6A, barrier 606 comprises a sharpedge at a right-angled surface configured to prevent helium from exitingthe cooling stage 600 through the low pressure pumping outlet.

Other embodiments of a barrier configured to prevent ⁴He creep are shownin the examples of FIGS. 6B, 6C, 6D, and 6E. In the example of FIG. 6B,a ring 608 with a knife edge 609 is configured to prevent ⁴He fromexiting through the ³He outlet. In some embodiments, the ring 608 mayinclude a heating device (e.g., a resistive heater or any other suitableheater) configured to cause the creeping ⁴He to leave the superfluidphase, thereby mitigating creep.

In the examples of FIGS. 6C, 6D, and 6E, vertical knife edges are usedto prevent ⁴He from exiting through the ³He outlet. In some embodiments,and in the example of FIG. 6C, the knife edge 610 is on the exteriorcircumference of the outlet pipe. Alternatively, as shown in the exampleof FIG. 6D, the knife edge 611 may be beveled on the interiorcircumference of the outlet pipe. Further, as shown in the example ofFIG. 6E, the knife edge 612 may be beveled on both the exterior andinterior circumferences of the outlet pipe.

In some embodiments, and as shown in the example of FIG. 6F, the ³Heoutlet may include a p-trap 621 configured to collect ⁴He 622 at a lowersurface of the p-trap 621. The ³He outlet may include a normal leak or asuperleak 624 configured to allow normal or superfluid ⁴He to exit thep-trap 621, and a pump 626 (e.g., a fountain pump) may transport the ⁴Heaway from the p-trap 621. In some embodiments, a further barrier 627 maybe present on an interior surface of the outlet pipe. For example, thebarrier 627 may be configured as a ring. In some embodiments, thebarrier 627 may include a heating device to further prevent ⁴He fromexiting the outlet.

The inventors have recognized and appreciated that nanomaterials canprovide advantages compared to conventional sintered metal powders(e.g., silver and/or copper powder) used in typical discrete heatexchangers. Accordingly, the inventors have developed nanomaterial heatexchangers that provide efficient heat exchange because of thenanomaterials' large surface area, high mechanical contact strength, andgood neck growth between nanowires.

Typical discrete heat exchangers are commonly made out of sintered metalpower (e.g., silver and/or copper powder). An example of such sinteredparticulates is shown in FIG. 7A. To have efficient heat exchange at thesub-Kelvin temperatures, however, a number of factors must be true withregards to the heat exchanger materials. The heat exchange materialshould have a large surface area, provide high mechanical and/or thermalcontact between the liquid helium and the heat exchange material, allowfor good neck growth, and provide space inside the heat exchangematerial for the liquid helium to move through the heat exchanger.

FIG. 7B shows an image of a nanomaterial comprising nanowires for use ina heat exchanger. FIG. 7C shows an image of a nanomaterial comprisingnanoclusters for use in a heat exchanger. FIG. 7D shows images ofnanomaterials comprising different examples of nanopellet shapes for usein a heat exchanger, in accordance with some embodiments describedherein. These nanomaterials may be implemented in dilution refrigerator100 in discrete heat exchanger 120 and/or in a block heat exchanger(e.g., present in the mixing chamber 122). It should be appreciated thatFIGS. 7B-7D show examples of nanomaterial shapes, but that embodimentsof nanomaterial for use in a discrete heat exchanger are not so limited.For example, the nanomaterial may alternatively be a nanofoam, nanotube,and/or any other suitable nanoshape.

In some embodiments, such a nanomaterial-based heat exchanger may beformed by bonding the nanomaterial through sintering. For example, thenanomaterial may be formed as a chemical precipitate and/or byelectronic deposition or electroplating techniques. A substrate with arough surface (e.g., comprising nucleation sites) may be provided forthe nanomaterial to be grown on or adhered to. In some embodiments, theheat exchanger may be produced under heat and/or compression. Thenanomaterial may be held in compression during the sintering process toform the nanowire heat exchanger. In some embodiments, the substrate maybe patterned with macroscopic structures (e.g., a lattice or series ofposts). In some embodiments, the substrate may be a tube, and thenanomaterial may be adhered to the interior or exterior surface of thetube. In some embodiments, the substrate may be formed of a materialwith a lower thermal conductivity than the nanomaterial adhered to thesubstrate.

In some embodiments, the nanomaterial may be formed out of one of aselection of vacuum-compatible materials including but not limited tocopper, silver, vacuum-compatible polymers, carbon, and/or carbon fiber.For example, the nanomaterial may be nanowires comprising at least oneof copper nanowires, silver nanowires, gold nanowires, platinumnanowires, polymer nanowires, carbon nanowires, and/or carbon fibernanowires.

II. Improved Vibration Isolation

Many experiments conducted at sub-Kelvin temperatures are sensitive tovibrational noise both from the surrounding environment and the dilutionrefrigerator's cooling system pumps and components. Additionally, atsub-Kelvin temperatures, mechanical vibrations can generate a heat load,reducing the cooling power of the dilution refrigerator or producingtribo-electric noise on electrical inputs and/or outputs of the dilutionrefrigerator. The inventors have recognized and appreciated thatimproved vibration isolation can improve the cooling power and otherperformance characteristics (e.g., magnetic flux disruption) of adilution refrigerator. Accordingly, the inventors have developedvibration isolation components configured to mechanically decouple thelower thermal stages 108 d-108 f from the upper thermal stages 108 a-108c.

FIG. 8A shows another schematic diagram of dilution refrigerator 100including mechanical elements configured to provide vibration isolation,in accordance with some embodiments described herein. The vibrationisolation elements include a first suspension system 832, at least onesecond suspension system 840, and a third suspension system 834.

In some embodiments, the first suspension system 832 may be configuredto suspend the first thermal stage 108 a, the second thermal stage 108b, and/or the third thermal stage 108 c from the top surface of theouter vacuum chamber 106. The first suspension system 832 may includeone or more rods configured to rigidly couple the first, second, and/orthird thermal stages 108 a-108 c to the top surface of the outer vacuumchamber 106. The rods may be formed of a material having a high springconstant. For example, the rods may be formed of carbon fiber and/orstainless steel.

In some embodiments, the second suspension system 840 may be configuredto independently suspend the fourth thermal stage 108 d, the fifththermal stage 108 e, and/or the sixth thermal stage 108 f from the topsurface of the outer vacuum chamber 106. This independent suspension ofthe lower thermal stages 108 d-108 f vibrationally isolates the lowerthermal stages 108 d-108 f from the upper thermal stages 108 a-108 c,thereby improving the vibration isolation of the lower thermal stages108 d-108 f.

In some embodiments, the second suspension system 840 may include one ormore springs 842, rods 843, and/or connectors 844. While the example ofFIG. 8A shows only one second suspension system 840, it should beappreciated that multiple second suspension systems 840 may be used tosuspend the lower thermal stages 108 d-108 f, in some embodiments. Forexample, there may be two, three, or four such second suspension systems840, in some embodiments.

In some embodiments, the springs 842 may be configured to provide aconstant spring tension under different loads (e.g., for differentdampened masses hanging from the springs 842). An example of a spring842 is shown in FIG. 8B. The springs 842 may be leaf suspension springs,in some embodiments, and may include an upper flexure 842 a separatedfrom a lower flexure 842 b by rigid portions 842 c. The springs 842 mayflex through the flexing of upper and lower flexures 842 a, 842 b,providing vibrational isolation to the lower thermal stages 108 d-108 falong the Z axis (e.g., perpendicular to a plane of the floor supportingthe dilution refrigerator 100). In some embodiments, a spring constantof the spring 842 may be determined by pre-tensioning the upper and/orlower flexures 842 a, 842 b. Alternatively or additionally, the springconstant of the spring 842 may be determined by changing a length of theupper and/or lower flexures 842 a, 842 b (e.g., having a lower springconstant for longer lengths of flexures 842 a, 842 b).

In some embodiments, the springs 842 may be coupled to the third thermalstage 108 d by rods 843. The rods 843 may be soft rods having a lowspring constant. For example, the rods 843 may be formed out of apolymer (e.g., DELRIN), in some embodiments. The rods 843, due to theirsoftness, may provide vibrational isolation to the lower thermal stages108 d-108 f in the X-Y plane (e.g., in a plane parallel to a plane ofthe floor supporting the dilution refrigerator 100 and perpendicular tothe Z axis).

In some embodiments, the connectors 844 may be arranged in a triangularconfiguration to provide stability to the suspension of the fourththermal stage 108 d. The connectors 844 may be made of materialsconfigured to have a high spring constant. For example, the connectors844 may be formed of stainless steel and/or carbon fiber.

In some embodiments, the third suspension system 834 may be configuredto suspend the fifth thermal stage 108 e and the sixth thermal stage 108f from the fourth thermal stage 108 d. In this manner, the three lowerthermal stages 108 d-108 f may all be suspended from the top surface ofthe vacuum chamber 106 using the second suspension system 840. The thirdsuspension system 834 may include one or more rods configured to rigidlycouple the fifth and/or sixth thermal stages 108 e, 108 f to the fourththermal stage 108 d. The rods may be formed of a material having a highspring constant. For example, the rods may be formed of carbon fiberand/or stainless steel, in some embodiments.

It should be appreciated that while the example of FIG. 8A shows thefirst and third suspension systems 832, 834 as being formed out of rods,in some embodiments, the first and/or the third suspension system 832,834 may be formed out of flexible springs, as aspects of the presentdisclosure are not limited in this respect. Additionally, it should beappreciated that while the example of FIG. 8A shows the secondsuspension system 840 as being formed out of springs, in someembodiments, the second suspension system 840 may be formed out of rods,as aspects of the present disclosure are not limited in this respect.

III. Integrated Dilution Refrigerator

Conventional dilution refrigerator technology often requires largeamounts of space and expensive supporting infrastructure such ascustom-built floating foundations, high ceilings, and/or access pits.These infrastructure requirements may reduce the scalability of quantumtechnologies that operate at low temperatures. As a non-limitingexample, the adoption of certain quantum computing technologies may belimited by the required use of large dilution refrigerators. Theinventors have recognized and appreciated that reducing the size andinfrastructure requirements of dilution refrigerators may enable thescalability of quantum technologies. The inventors have furtherrecognized that integrating dilution refrigerators with commercialcomputing infrastructure (e.g., commercial server infrastructure) canfurther enable the scalability of dilution refrigerators and associatedquantum technologies dependent on dilution refrigerators. Suchintegrated dilution refrigerators may be more easily integrated intotelecommunications networks, can use existing telecommunications heatremoval architectures, and integrate with fiberoptic networks andsystems.

FIG. 9 is a side view of an illustrative external support rack 950, inaccordance with some embodiments described herein. In some embodiments,the external support rack 950 may support the dilution refrigerator 100by suspending the dilution refrigerator 100 off of the floor below thedilution refrigerator 100. As shown in the example of FIG. 9 , theexternal support rack 950 may include arms 952 that are coupled toportions of the top surface of the vacuum chamber 106 by vibrationisolation components 954 to suspend the dilution refrigerator 100 off ofthe floor. In some embodiments, the vibration isolation components 954may be air pistons, electromagnetic dampeners, and/or springs.

In some embodiments, the external support rack 950 may include castors(not shown) configured to assist in transportation of the dilutionrefrigerator 100. The castors may be retractable such that the wheels ofthe castors are not in contact with the floor supporting the externalsupport rack 950 when the dilution refrigerator 100 is not beingtransported and/or is in operation.

In some embodiments, the external support rack 950 further includesfloor supports 958. Floor supports 958 may be configured to extend fromthe external support rack 950 when the dilution refrigerator 100 is notbeing transported. Floor supports 958 may extend from the externalsupport rack 950, for example, by the use of screws. The floor supports958 may be used to lift and/or level the external support rack 950 awayfrom the floor and/or to lift the castors of the external support rack950 off of the floor. In some embodiments, the floor supports 958 may beused to correct the positioning of the external support rack 950 in thecase of an uneven floor surface.

In some embodiments, the external support rack 950 may supportadditional components external to the outer vacuum chamber 106 ofdilution refrigerator 100. For example, the external support rack 950may house compressors, pumps, and/or cooling equipment configured tosupport the operation of the dilution refrigerator 100. Alternatively,these external components may be housed in an adjacent (e.g., adifferent) server rack-type container and/or support rack 950 than thedilution refrigerator 100, in some embodiments.

In some embodiments, the external support rack 950 may include elementsconfigured to provide tool-free assembly and/or disassembly of thevacuum chamber 106 and access to the experimental volume, in accordancewith some embodiments described herein. As shown in the example of FIG.9 , the vacuum chamber 106 includes three sections, a first section 106a, a second section 106 b suspended from the first section 106 a, and athird section 106 c suspended from the second section 106 b. It shouldbe appreciated that the technology described herein is not limited tothree sections, and that a vacuum chamber may have one, two, four, five,or six sections in some embodiments.

In some embodiments, the vacuum chamber 106 may have one or moresubstantially planar surfaces. In some embodiments, at least one of theone or more substantially planar surfaces may be disposed within a planeperpendicular to a plane of a floor supporting the dilutionrefrigerator. As shown in the example of FIG. 9 , the sections 106 a-106c may each have at least four substantially planar surfaces such that,when assembled, the vacuum chamber 106 is arranged to form a rectangularprism. In some embodiments, the vacuum chamber 106 may include twosubstantially planar surfaces disposed within a plane parallel to theplane of the floor and arranged to close the rectangular prism formed bythe surfaces of the sections 106 a-106 c. In some embodiments, and asdescribed below, the vacuum chamber 106 may have an opening accessibleby a door 1070 in at least one of the substantially planar surfaces.

In some embodiments, the three sections 106 a-106 c of the vacuumchamber 106 may be partially or fully removable in order to provideaccess to internal portions of the dilution refrigerator 100. Forexample, the three sections 106 a-106 c of the vacuum chamber 106 maycomprise removable panels (e.g., side panels, panels attached to aframe, etc.), in some embodiments. The three sections 106 a-106 c may beconfigured to allow a user of the dilution refrigerator 100 to be ableto remove the vacuum chamber 106 from the dilution refrigerator 100without needed a large clearance above or below the dilutionrefrigerator 100 (e.g., without needing high ceilings or a pitunderneath the dilution refrigerator 100).

In some embodiments, the external support rack 950 may include anintegrated lift 956 a configured to support the three sections 106 a-106c of the vacuum chamber during assembly, disassembly, and/or maintenanceof the dilution refrigerator 100. The integrated lift 956 a may beconfigured to raise and/or lower the sections 106 a-106 c of the vacuumchamber. For example, the integrated lift 956 a may be configured toraise and/or lower arms 956 b configured to support portions (e.g., theflanges) of the three sections 106 a-106 c. In some embodiments, theintegrated lift 956 a may be operated manually (e.g., using screwsand/or cables). In some embodiments, the integrated lift 956 a may beoperated using an electronically-operated machine (e.g., pneumatic orhydraulic devices).

In some embodiments, the external support rack 950 may include one ormore carts 957. The carts 957 may be configured to receive one or moreof the sections 106 a-106 c when lowered manually or by using theintegrated lift 956 a. For example, the integrated lift 956 a may beused to lower the third section 106 c onto a cart 957. Thereafter, thethird section 106 c may be transported using the cart 957 alongdirection C to provide a user of the dilution refrigerator 100 spaceunder the interior components of the dilution refrigerator 100.

In some embodiments, the integrated lift 956 a may be removably coupledto the external support rack 950. For example, the integrated lift 956 amay be slidably removable from the external support rack 950 (e.g.,sliding horizontally outward along the direction C). Removal of theintegrated lift 956 a may be desired to provide the user with extraspace (e.g., during maintenance of the dilution refrigerator 100).

In some embodiments, the three sections 106 a-106 c of the vacuumchamber 106 may be suspended from one another by integrated clampsand/or cams. Such integrated clamps and/or cams may be configured toenable a user to unclamp or clamp two sections of the three sections 106a-106 c together without the use of any additional tools. FIGS. 10A-10Cillustrate an example of an integrated cam 1060, with FIG. 10B showingthe integrated cam 1060 in an open position and FIG. 10C showing theintegrated cam 1060 in a closed position.

In some embodiments, the integrated cam 1060 includes a handle 1062 thatenables a user to clamp or unclamp two sections of the three sections106 a-106 c together or apart. The handle 1062 is coupled to two latches1064 that are configured to connect to bars 1066. The handle 1062 andlatches 1064 are hingedly coupled to a section of the vacuum chamber 106by cams 1068, which provide the requisite range of motion to performclamping and unclamping motions.

In some embodiments, a compressive layer may be included at theconnection points between the three sections 106 a-106 c of the vacuumchamber 106 to ensure a proper vacuum-safe seal. For example, a rubberO-ring, copper or indium gasket, or other vacuum-safe compressive layermay be placed between sections 106 a-106 c.

Returning to FIG. 10A, in some embodiments, the vacuum chamber 106 mayinclude an external opening to provide access to an internal volumewithin the dilution refrigerator 100. For example, the opening mayprovide access to a sample stage or experimental volume of the dilutionrefrigerator 100, or any other interior portion of the dilutionrefrigerator 100. In some embodiments, the opening may be sealed by ahermetic seal. In some embodiments, the opening may be sealed by a door1070, as shown in the example of FIG. 10A. The door 1070 may be sealed,for example, using a hinge and/or a clamp that may be manually engagedand disengaged. In some embodiments, the door 1070 may be coupled to theopening by a load lock.

In some embodiments, the door 1070 may provide access through all of theinner radiation shields (not shown, and which may be thermally coupledto one or more of the thermal stages 108 a-108 f) of the dilutionrefrigerator 100 to allow a user to access the experimental volume. Forexample, a portion of the inner radiation shields (not shown) may becoupled to the door 1070 such that when a user opens the door 1070, theinner radiation shields slide or otherwise move to provide the useraccess with the interior portion of the dilution refrigerator. In someembodiments, a portion the inner radiation shields may be removablethrough the door 1070 and/or slidable through the door 1070.

In some embodiments, the external support rack 950 may be configured tobe integrated with a server rack-type container. For example, theexternal support rack 950 may be configured to integrate the dilutionrefrigerator 100 with commercial server rack infrastructure (e.g.,server racks). In some embodiments, the external support rack 950 may beconfigured to integrate the dilution refrigerator 100 with 19-inchserver racks.

In some embodiments, the external support rack 950 and dilutionrefrigerator 100 may be housed within an outer housing. An example of anouter housing 1100 is illustrated in FIG. 11 . In some embodiments, theouter housing 1100 may include an integrated horizontal surface 1110.For example, the integrated horizontal surface 1110 may be used as adesk or support surface when the user interacts with the dilutionrefrigerator. The integrated horizontal surface 1110 may be configuredto be stowed by folding (as shown in the example of FIG. 11 ) or slidingaway when not in use. In some embodiments, the outer housing 1100 mayfurther include one or more storage locations (e.g., drawers, shelves)for the storage of related parts and/or tools for maintenance of thedilution refrigerator 100.

In some embodiments, the outer housing 1100 may further include a door1125 providing access through an opening 1120 to the experimental volumeof the dilution refrigerator 100. For example, the door 1125 may open toprovide access to the experimental volume through the vacuum chamber 106and the radiation shields inside of the vacuum chamber 106. In someembodiments, the vacuum chamber exterior and/or the radiation shieldsmay be coupled to the door 1125 such that when a user opens the door1125, the user opens the vacuum chamber exterior 106 and/or theradiation shields. In some embodiments, the radiation shields mayalternatively be slidably and/or hingedly movable such that the user maymove the radiation shields such that they no longer block access to theexperimental volume as needed.

In some embodiments, the housing 1100 may further be configured toperform sound dampening. For example, the housing 1100 may include sounddampening materials to perform passive sound dampening. Alternatively oradditionally, the housing 1100 may include audio equipment (e.g.,speakers) configured to provide active sound dampening through theemission of destructive interference of the sounds generated byfunctional components of the system.

IV. Inverted Dilution Refrigerator

Conventionally, dilution refrigerators are oriented such that warmerthermal stages are positioned towards the top of the system with thethermal stages getting increasingly colder as the ³He/⁴He mixtureprogresses to the bottom of the dilution refrigerator. The inventorshave recognized and appreciated that an inverted geometry, with thecoldest stage disposed at the top of the system (e.g., furthest from thefloor) may simplify the operation and use of a dilution refrigerator bymaking the experimental volume more accessible to a user and offerimproved thermodynamic qualities compared to a conventional dilutionrefrigerator. Accordingly, the inventors have developed an inverted, drydilution refrigerator.

FIG. 12A is a schematic diagram of an inverted dilution refrigerator1200, in accordance with some embodiments described herein. The inverteddilution refrigerator 1200 may include a pump system 1202 configured tocirculate the ³He/⁴He mixture through the dilution refrigerator 1200.The inverted dilution refrigerator 1200 also may include a cryocooler1204. The cryocooler may be coupled to a cryocooler support (notpictured) as described in connection with cryocooler support 103 herein.

In some embodiments, the inverted dilution refrigerator 1200 may includean outer vacuum chamber 1206 and a series of thermal stages 1208 a-1208f disposed inside of the outer vacuum chamber 1206. The series ofthermal stages 1208 a-1208 f may be held at same or similar temperaturesas the thermal stages 108 a-108 f described in connection with FIG. 1herein.

In some embodiments, the inverted dilution refrigerator 1200 may includean opening in the outer vacuum chamber 1206 and/or through the innerradiation shields to provide ease of access to the coldest stage of theinverted dilution refrigerator 1200. In some embodiments, the openingmay comprise hermetic seals and/or an opening mechanism 1225 that maywithstand the vacuum within the outer vacuum chamber when the inverteddilution refrigerator 1200 is in operation. For example, the openingmechanism 1225 may include a hinged door and/or a removable panel.

In some embodiments, the inverted dilution refrigerator 1200 may includea number of components arranged along the length of the dilutionrefrigerator (e.g., from within the vacuum chamber 1206 to within thesixth thermal stage 1208 f). The components may be arranged with thecoldest thermal stage, the mixing chamber 1222, disposed above warmerthermal stages (e.g., the still 1214, impedance stages 1212 and 1216,heat exchangers 1218, 1219, 1220, etc.).

In some embodiments, the inverted dilution refrigerator 1200 includes ade-mixing chamber 1224 coupled to the mixing chamber 1222. In someembodiments, the de-mixing chamber 1224 may be thermally coupled to themixing chamber 1222 by a heat exchanger 1223 (e.g., a co-flow heatexchanger). The de-mixing chamber 1224 may be fluidly connected to themixing chamber 1222 such that ³He may be transported from the de-mixingchamber 1224 to the mixing chamber 1222 to provide additional cooling tothe mixing chamber 1222. The de-mixing chamber 1224 may additionallyhave ⁴He injected into the de-mixing chamber 1224 to provide a co-flowof ³He and ⁴He in order to mitigate a concentration gradient formingbetween the still 1214 and the mixing chamber 1222.

FIGS. 12B and 12C are schematic diagrams of exemplary internalcomponents of an inverted dilution refrigerator, in accordance with someembodiments described herein. It should be appreciated that theexemplary components of FIGS. 12B and 12C could be implemented withininverted dilution refrigerator 1200 of FIG. 12A (e.g., within the thirdthermal stage 1208 c).

As shown in the example of FIG. 12B, in some embodiments the inverteddilution refrigerator may include a ⁴He line 1226 configured totransport ⁴He from the still 1214 to the de-mixing chamber 1224. In someembodiments, the ⁴He line 1226 may include a pump 1228 configured toassist in the transportation of the ⁴He to the de-mixing chamber 1224.The pump 1228 may be, for example, a fountain pump in some embodiments.Alternatively or additionally, the ⁴He line 1226 may include additionalheat exchangers to cool the ⁴He as it travels to the de-mixing chamber1224.

As shown in the example of FIG. 12C, in some embodiments, the inverteddilution refrigerator may include a heat exchange stage 1230 configuredto cool the incoming ³He/⁴He mixture prior to the primary impedancestage 1212. As should be appreciated from the description of FIG. 1 ,such a configuration may increase the efficiency of the first impedancestage and/or eliminate or reduce the need for pressurization of theincoming ³He/⁴He mixture.

V. Distributed Cooling

Dilution refrigerators generally include an integrated cryocooler (e.g.,such as a pulse tube or a Gifford-McMahon cryocooler) to pre-cool the³He/⁴He mixture gas below 5 K. Conventionally, a dilution refrigeratoris paired with at least one of these cryocoolers, and dilutionrefrigerators do not share cooling systems. Such small-scale dilutionrefrigeration systems typically rely on low-power cryocooling systemsthat are relatively inefficient (e.g., requiring more power for eachWatt of cooling power at 4 K) in comparison to larger, higher-powercryocooling systems. The inventors have recognized and appreciated thata single, high-efficiency cooling system may be thermally coupled tomultiple cryogenic systems such as dilution refrigerators to distributethis first stage of cooling across multiple cryogenic systems. Suchdistributed cooling therefore allows for increased cooling efficiencyacross multiple cryogenic systems.

FIG. 13 is a schematic diagram of a distributed cooling system 1300, inaccordance with some embodiments described herein. The distributedcooling system 1300 may include multiple housings 1305. In someembodiments, the housings 1305 may be server rack-type containers (e.g.,commercial server rack infrastructure, 19-inch server racks).

As shown in the example of FIG. 13 , each housing 1305 may contain acooling system 1310 or a cryogenic device 1320. It should be appreciatedthat cryogenic devices 1320 and/or cooling systems 1310 may be groupedwithin housings 1305, in some embodiments. It should further beappreciated that while FIG. 13 shows three cryogenic devices 1320coupled to the cooling system 1310, in some embodiments, there may betwo, four, ten, or many tens of cryogenic devices 1320 coupled to thecooling system 1310, as aspects of this disclosure are not so limited.

In some embodiments, the cooling system 1310 may be a cryocooling systemconfigured to cool a first stage of the cryogenic devices 1320 to atemperature of at least 5 K and/or to a temperature of approximately 4-5K. In some embodiments, the cooling system 1310 may be a pulse tube. Forexample, the cooling system 1310 may be a pulse tube, a helium liquefiersystem, and/or a Brayton cryocooler.

In some embodiments, the cooling system 1310 may be thermally coupled tomultiple cryogenic devices 1320. Cooling may be distributed to cryogenicdevices 1320 from cooling system 1310 by cooling line 1312.Additionally, heat may be returned from the cryogenic devices 1320 tocooling system by return 1314. The cooling line 1312 and/or return 1314may be lines configured to transfer liquid and/or gaseous helium. Forexample, the cooling line 1312 and/or return 1314 may be pipes that arevacuum insulated to maintain the temperature of the transported helium.In some embodiments, the cooling line 1312 and/or return 1314 may befill lines, heat pipes (e.g., traditional and/or pulsed heat pipes),and/or a superfluid loop.

In some embodiments, the cryogenic devices 1320 may include any suitablerefrigeration system configured to reach temperatures at or below 5 K.In some embodiments, cryogenic devices 1320 may include one or moredilution refrigerators (e.g., dilution refrigerator 100 as describedherein, configured to reach temperatures below 1 K). Alternatively oradditionally, it should be appreciated that cryogenic devices 1320 mayinclude cryogenic systems other than dilution refrigerators (e.g.,microscopy systems such as scanning tunneling microscopy or atomic forcemicroscopy systems, ³He refrigeration systems, superconducting CMOSsystems, etc.).

In the embodiment shown in FIG. 14 , the computer 1400 includes aprocessing unit 1401 having one or more processors and a non-transitorycomputer-readable storage medium 1402 that may include, for example,volatile and/or non-volatile memory. The memory 1402 may store one ormore instructions to program the processing unit 1401 to perform any ofthe functions described herein. The computer 1400 may also include othertypes of non-transitory computer-readable medium, such as storage 1405(e.g., one or more disk drives) in addition to the system memory 1402.The storage 1405 may also store one or more application programs and/orresources used by application programs (e.g., software libraries), whichmay be loaded into the memory 1402.

The computer 1400 may have one or more input devices and/or outputdevices, such as devices 1406 and 1407 illustrated in FIG. 14 . Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, the input devices 1407may include a microphone for capturing audio signals, and the outputdevices 1406 may include a display screen for visually rendering, and/ora speaker for audibly rendering, recognized text.

As shown in FIG. 14 , the computer 1400 may also comprise one or morenetwork interfaces (e.g., the network interface 1410) to enablecommunication via various networks (e.g., the network 1420). Examples ofnetworks include a local area network or a wide area network, such as anenterprise network or the Internet. Such networks may be based on anysuitable technology and may operate according to any suitable protocoland may include wireless networks, wired networks or fiber opticnetworks. Such networks may include analog and/or digital networks.

Various aspects of the embodiments described above may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B,” when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

The terms “approximately,” “about,” and “substantially” may be used tomean within ±20% of a target value in some embodiments, within ±10% of atarget value in some embodiments, within ±5% of a target value in someembodiments, within ±2% of a target value in some embodiments. The terms“approximately,” “about,” and “substantially” may include the targetvalue.

Having thus described several aspects of at least one embodiment, it isto be appreciated that various alterations, modifications, andimprovements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure and are intended to be within the spirit and scope ofthe principles described herein. Accordingly, the foregoing descriptionand drawings are by way of example only.

What is claimed is:
 1. A dilution refrigerator comprising: an outervacuum chamber comprising at least one substantially planar surface; andan opening in the at least one substantially planar surface configuredto provide access to an interior of the outer vacuum chamber.
 2. Thedilution refrigerator of claim 1, further comprising a sample stagehoused within the outer vacuum chamber, wherein the opening isconfigured to provide access to the sample stage through the outervacuum chamber.
 3. The dilution refrigerator of claim 2, furthercomprising one or more radiation shields housed within the outer vacuumchamber, and wherein portions of the one or more radiation shieldsproximate the sample stage are slidable and/or removable to provideaccess to the sample stage.
 4. The dilution refrigerator of claim 1,wherein the at least one substantially planar surface comprises a firstsurface disposed within a plane perpendicular to a plane of a floorsupporting the dilution refrigerator.
 5. The dilution refrigerator ofclaim 1, wherein the at least one substantially planar surfacecomprises: first surfaces disposed within a plane perpendicular to aplane of a floor supporting the dilution refrigerator; and secondsurfaces disposed within a plane parallel to the plane of the floor,wherein the first surfaces and the second surfaces are arranged as arectangular prism.
 6. The dilution refrigerator of claim 1, wherein theopening comprises a hermetic opening.
 7. The dilution refrigerator ofclaim 1, wherein the opening comprises a hinged door.
 8. The dilutionrefrigerator of claim 7, further comprising one or more radiationshields housed within the outer vacuum chamber, and wherein portions ofthe one or more radiation shields proximate the hinged door are slidableand/or removable to provide access to the interior of the outer vacuumchamber.
 9. The dilution refrigerator of claim 7, wherein the openingfurther comprises a load lock.
 10. The dilution refrigerator of claim 1,wherein the outer vacuum chamber comprises: a first section; and asecond section suspended from the first section.
 11. The dilutionrefrigerator of claim 10, wherein the first section is coupled to thesecond section by integrated clamps and/or cams.
 12. The dilutionrefrigerator of claim 10, wherein the first and/or second section of theouter vacuum chamber are configured to be at least partially removedfrom the dilution refrigerator to provide access to an interior of theouter vacuum chamber.
 13. The dilution refrigerator of claim 10, furthercomprising an external system configured to lift and/or lower the firstand/or second section of the outer vacuum chamber.
 14. The dilutionrefrigerator of claim 13, wherein the external system comprisespneumatic and/or hydraulic devices configured to lift and/or lower thefirst and/or second sections.
 15. The dilution refrigerator of claim 13,wherein the external system comprises a screw mechanism configured tolift and/or lower the first and/or second sections.
 16. The dilutionrefrigerator of claim 1, wherein the outer vacuum chamber is configuredto fit within a server rack-type container.
 17. The dilutionrefrigerator of claim 16, wherein the server rack-type container isconfigured to integrate with commercial server rack infrastructure. 18.The dilution refrigerator of claim 16, wherein the server rack-typecontainer is a 19-inch server rack.
 19. The dilution refrigerator ofclaim 17, wherein the server rack-type container comprises an externalhousing comprising an integrated horizontal surface.
 20. The dilutionrefrigerator of claim 19, wherein the integrated horizontal surface isconfigured to be stowed when not in use.